The present invention relates to an improved cooling system employed for cooling of hot, aqueous caustic solutions, such as obtained in the evaporative concentration of electrolytically produced dilute caustic solutions. In the production of caustic solutions by electrolysis of aqueous brines, generally a relatively dilute caustic solution is obtained. This aqueous caustic solution frequently has only a 15-20% caustic content and is unsuited for most uses. Thus, it needs to be concentrated to a commercially acceptable concentration, usually up to a 40-55% caustic level. Evaporative concentration is the most commonly employed process for the removal of a desired portion of the water content of the solution and this is generally accomplished by heating the dilute caustic solution to about 100.degree.-150.degree. C. The concentrated, hot caustic exiting from the evaporator is generally cooled to a temperature at which it can be handled, but more importantly, cooling is applied to reduce the solubility of caustic-soluble impurities. At lower temperatures, a significant quantity of the dissolved impurities will precipitate and can then be removed from the caustic solution by conventional solid-liquid separation methods.
Cooling of the hot caustic solution has been generally accomplished in agitated, open vessels or tanks equipped with coils through which a cooling medium, usually water, is conducted. For optimum operating efficiency, several series-connected tanks or vessels have been used and the caustic solution temperature has been gradually reduced from vessel to vessel until it reached the desired range.
In the prior art cooling system, such as described above, rapid precipitation of a portion of the dissolved impurities occurs in the first cooling tank with consequent scale formation on the cooling coil surfaces. The scale on the coil surfaces has been found to rapidly increase to such an extent that heat transfer from the coil surfaces to the hot caustic solution is seriously impeded and the cooling rate is decreased to an unacceptable low degree. At this point under usual plant practice, this tank is bypassed and after removal of a significant portion of its caustic content, water is added to the tank. This will cause the dissolution of the deposited scale with the simultaneous generation of a dilute caustic solution. The solution thus obtained is generally recycled to the evaporator to remove the added water content and to recover the caustic values.
In addition to the rapid scale formation in the first vessel, contact of the hot caustic with CO.sub.2 in the air causes carbonate formation which not only contaminates the caustic solution to be cooled, but also reduces the yield of caustic. Thus, it can be seen that the prior art cooling system employed for the cooling of caustic solutions presents many problems which renders the process not only economically undesirable but also energy intensive as a result of the frequent need of recycling and evaporating the salt-containing dilute caustic from the cooling vessel.
It has now been found that these difficulties can be readily overcome by employing a novel cooling system consisting of at least one closed-loop cooling circuit in which the hot caustic solution to be cooled is circulated under pressure in the absence of air. In this system, the rate of caustic flow can be controlled in such a manner as to obtain a desired degree of cooling at a significantly reduced rate of scaling. Due to the use of a closed, air-free system, reaction of the hot caustic with the CO.sub.2 content of the atmosphere is prevented, thus the yield and quality of cooled caustic is improved. Further, since the rate of cooling in the heat exchanger employed is controllable, the rate of scale formation can also be controlled, and, consequently, the frequency of scale dissolution from the heat exchanger surfaces can be reduced which results in evaporation energy savings.